This invention relates generally to optical telecommunications networks, and in particular to methods and apparatus for installing fiber optic communication cables in ducts and conduits, including underground ducts and guide tubes that channelize the underground ducts.
Various factors must be considered when a fiber optic cable is installed in a protective duct. Some major concerns are the ease of installation and reduction in the amount of time needed to install the cable. Generally, it is desirable to install the longest continuous length of cable possible to reduce the number of splices needed for the cable run. Splices are time consuming to make and incur considerable installation costs. Moreover, it is not desirable to have a large number of splice joints in view of the relatively substantial signal damping caused by each joint in proportion to the total signal damping of the overall signal path.
Protective cable ducts have been channelized in an effort to satisfy these concerns. For this purpose one or more guide tubes, whose interior may have a lower coefficient of friction than the existing duct, are installed in the existing protective duct, thereby establishing separate channels in which cable, optionally at a later time, can be pulled through the protective duct over a greater length.
The pulling of fiber optic cables has now largely been replaced by a combination of blowing and synergetic pushing of the cables, e.g. such as described in U.S. Pat. Nos. 4,850,569 and 4,934,662 to Griffioen et al. This method is being used now for a variety of cables and ducts, from small (4 mm optical cables in 7/5.5 mm guide tubes, and currently 1.6 mm optical fiber cables in 4/3.2 mm guide tubes) to large (35 mm copper balanced cables in 63/50 mm ducts).
The theory of this pushing and blowing technique is described in EP 0734 109 B1 (Griffioen). According to this theory, cables with only a little play in the duct can be installed over long distances. Although the stiffness of the cable contributes more to the friction when passing bends and windings (undulations) in the duct trajectory, pushing becomes more efficient because the cable has less play to develop buckling. Even so, frictional effects have limited the installation lengths attainable by pushing and blowing techniques. Improvements in cable lubrication have been made to overcome those frictional limitations.
During the early development of pushing/blowing installation, cable lubrication was done by leading the cable through a box in contact with a lubricant, e.g., paraffin oil, before entering the cable blowing equipment and the protective duct. That conventional method has two limitations:
(a) The pusher wheels or capstans of most blowing equipments (most of them with the pusher wheels outside the pressurized space) slip over the cable when the cable is wet with lubricant.
(b) The seals from the cable inlet to the pressurized space scrape or blow away most of the lubricant.
For these reasons the method of lubricating a cable before it is launched has been abandoned by most operators. Instead, the duct is now lubricated before the cable is inserted. This is done by pouring a proper amount of lubricant into the duct, putting a foam plug behind and blowing the plug with lubricant through the duct. This method turned out to be a reliable and satisfactory way of lubricating conventional large diameter ducts. Another way to lubricate is pre-lubrication of the duct by the manufacturer (usually during the extrusion process).
For use in telecommunications access networks, e.g., “Fiber To The Home” (FTTH) systems with bundles of mini-tubes have been developed, for example as described in U.S. application Ser. No. 09/571,157 entitled “Installation of Guide Tubes in a Protective Duct,” assigned to the owner of the present application and incorporated herein by reference. Here the mini-tubes become smaller and smaller, making it more difficult to lubricate with foam-plugs. More importantly, blowing many cables from one point to many different end-locations (houses or offices), and lubricating them with a blown foam plug every time, is rather impractical. Pre-lubricated ducts (by the manufacturer) have turned out not to be always reliable. A solution to these problems is given by the present invention as follows.